US2391523A - Method for casting rotors - Google Patents

Description

Dec. 25, c E. SQRENSEN 2,391,523

METHOD FOR CASTING ROTORS Filed March 31, 1941 /7 A M C? 5 M INVENTOR I BY ITNE My 7 W14, Jill ATTUH/VEYfi.

Rhiififittd. 13cc. 25, 1945 Charles E. Sorensen, Detroit, Micla, asslgnor to Ford Motor Company, Dearbom, Micln. a corporation of Delaware Application March 31, 1941. semi No. 336,033

3 Claims.

This invention relates to the manufacture of cast articles and, more specifically, to methods and molds for use in the centrifugal casting of various articles. I

It will be understood that by centrifugal casting' is meant the process in which molds are rotated for the purpose of distributing molten metal therein by centrifugal force. This results in an improved metallurgical structure; and, in addition, permits the casting of objects which could not be made by prior methods of casting.

In connection with such centrifugal casting, the use of so-called permanent molds has been quite common. These molds are constructed of metal or other substances which are more or less green sand or core molds used in the general casting process and which may be used but once. 7

While these molds may be used a number of time before being damaged by the hot metal, their useful life is limited and considerable effort has been directed to obtaining suitable metals and developing such designs as would permit a longer use of such molds. It is obvious that the more times the mold may be used, the greater will be the economy in the process. It is a further consideration that such molds may be repaired, or

damaged parts replaced, easily and cheaply. Apart from this, it is necessary that such a mold be easily assembled or set up preliminary to pourirg, and that it be easily disengaged from the cast article after the poured metal has solidified.

The foregoing, as applying both to the centrifugal casting process and the use of permanent molds, represents matters which have been the subject of considerable attention in the prior art. Thus, the general method of centrifugal casting is to set up a mold on a rotatable table, rotate the table at a suitable speed, and then to pour molten metal into the rotating mold. The centrifugal force imparted to the molten metal by the rotating mold is sufiicient to force the metal throughout the said mold and will insure a particularly dense structure in the outer parts of such casting. This process has been found adapted to such articles as car wheels, gear blanks and the like in which a high peripheral density is desired. Indeed, when centrifugal casting is used it is found that articles which were formerly thought to be necessarily forged or machined can be made by casting. In such instances, the process results in a substantial economy in both time and material.

This general method does have limitations and has been found unsuitable for articles such as are supercharger is shown.

is designed for use in an aircraft supercharger in described hereinafter, for which.the method of this invention is practicable.

As an illustration of the type of work to which this process may be applied, the impeller 01' a This particular impeller which it operates at extremely high rotational speed. Up to now. it was thought necessary to form these impellers by forging or machining.

This is a lengthy process; and, moreover, these methods necessarily limit the curvatures and the clearances which may be used. On the contrary, in using centrifugal castings, I find that this-impeller can be made from a high-strength aluminum alloy, that there is no limitation upon the shape of the blades as no machining is necessary, and that they may be turned out easily and quickly. It will be noted that the vanes of the impeller are of very thin section and it is here that this method of centrifugal casting has a particular advantage.

Considerable effort has also been expended in improving permanent molds from the standpoint of the metal used, the design, suitable facing agents to prevent ,the adherence of the metal to them and the easy stripping from the cast article. However, it is obvious that in any mold which is alternately subjected to high and low temperatures and which will, as a consequence, expand and contract with each use, failure must necessarily occur eventually. In addition, such molds were generally designed so as to have one surface in contact with the metal and others in contact with the atmosphere. This also set up stresses in the molds during the pouring operation and contributed to their failure. In still other designs, the problem of the expansion of the mold presented serious difiiculties. The mold of this invention, whichis also shown as applying to the impeller, avoids these difliculties and defects.

An advantage of the process of this invention is that by following it, articles may be cast satisfactorily which heretofore have been considered unsuited for such procedure. It is a further advantage that such articles may be produced more quickly and more cheaply than-heretofore. It is a further object of this casting process to extend the benefits of centrifugal casting to articles of relatively small section and to the use of the lighter alloys therein.

An object of the mold of this invention is to provide a mold of the permanent type which may be eadily assembled and disassembled. A further object is to construct a mold of interchangeable parts by which failure of any one of the parts may be immediately replaced without scrapping the mold as a whole. Another object is to design a mold in which proper compensation may be made for temperature changes and the expansion or contraction incident thereto. A further object is to design a mold in. which, insofar as possible, temperature stresses in the individual elements of the mold may be equalized.

With these and other objects in view; my invention consists in the arrangement, construction and combination of the various parts of my improved device, as described in this specification, claimed in my claims, and illustrated in the accompanying drawing, in which:

Figure 1 is a perspective view of one of the chills used in assembling the mold of this invention.

Figure 2 is a plan view'- on a reduced scale of the mold carrier showing a chill in place.

Figure 3 is a plan view of theimpeller as cast in the mold of this invention.

Figure 4 is an elevation of the impeller.

Figure 5 is a transverse section taken on the line 5 of Figure 2.

Figure 6 is a transverse section taken through the spinner unit used in making the centrifugal casting.

Referring now to the drawing, Figure 3 represents a plan view of an impeller I for a supercharger, having a lurality of blades-8 and a huh 9. In the pre ent case, it is cast from an aluminum alloy havinga melting point of about 1200 at a pouring temperature of about 1500. This impeller when so made, using this alloy, the mold of this invention, and the method outlined herein, will be suited for service in cases where heretofore only forged or machined construction has been satisfactory. Moreover, by casting it, such vane shapes as are desired may be obtained without limitation thereon due to ,the necessity of machining. Finally, these impellers may be produced cheaply and quickly and with a great degree of uniformity.

To accomplish this, the centrifugal casting apparatus shown in Figures 2, 5 and 6 is used in which l0 indicates a mold ring and ii a mold spinner. The former includes a dish-shaped recess lz'and a peripheral flange I 3. The mold spinner includes the complementary fiange ll, designed to seat within the peripheral flange l3 of the mold ring, and is provided with a centrallydisposed recess l5. The bottom of the mold spinner is formed with a collar i6 which fits over and is releasably secured to the spinning shaft I! as by the pin [8.

The chills from which the mold is built up are indicated generally at IS in Figure 1. tion of Figures 3 and 4 will indicate generally the required molding conformation of these chills,

since the molding portion of each chill occupies the space between two adjacent impeller blades, as indicated by the centerline 28 in Figure 4.

It will-be understood, of course, that the design of the impeller dictates'the conformation of the chill to be used; and that a chill to form any desired impeller design may be'made, the only limitation being that it may be removable from the impeller after casting.

Referring to Figure 1, it will be seen that the chill is has parallel upper and lower surfaces 20 and 2|, respectively, and each of these surfaces represents a part of a circular sector. The inner surface 22 of the chill conforms to the'exterior of the impeller hub between the vanes. The outer surface 23 of the chill is designed to bear against side of the chili}. is formed with a recess 2|; the depth of this recess conforms to the thickness of the impeller blade and its outer line 2! follows the configuration of the edge of the impeller blade. Ti e other side 21 does not have such arecesa.

As the'-,impeller under consideration has eighteen vanes, ,eighteen of these chills will be required. These chills are assembled in the mold ring as indicated in Figure 2. One of these chills is there shown in position, with its lower surface 2| resting on the base of the recess i2 and its outer surface 23 hearing against the wall of the recess. Another chili is then placed and its far side 21 bears against unrecessed portion of side 24 of the first chill. Other chills are then added one; after the other until all eighteen are placed,

' the recessed side 24 of the last chill placed contactifng the far side 21 of the first chill placed. The'centerline 28- on Figure '4 indicates the relationship of one such chill to the impeller as poured.

It is obvious that the chills now cooperate to form a mold for an impeller such as is shown. The outline formed by these assembled chills is indicated in Figure 5, although the "configuralI,-is inverted and placed over the mold ring in,

and clamps are applied around the edges of the spinner and the ring to hold them together. This assembly is then inverted, the spinner is re-e'ngaged with the shaft, and the mold is now ready to be poured.

It will be noted that an internally-flared pouring opening 29 is provided which is so shaped to prevent the molten metal from being thrown out of the mold ring during the casting process. The assembled mold is now preheated, it having been found that temperatures from 200 to 800 thereof are preferred before the actual pouring is begun. Molten metal is now introduced through the pouring opening 29 in quantities sufficient to fill the recess l5 and a substantial portion of the recess 30 formed by the assembled chills. As soon as this metal is poured, the rotating mechanism is started and the mold is rotated at a rate of from 700 to 1200 a. P. M. This rotation, of

Inspec course, impels the molten metal by centrifugal force to flow into and fill the interstices between the chills, thus forming the impeller blades. As a light alloy is used, the rotational speed is somewhat higher than used for iron. Those skilled in,this art will recognize that by introducing the molten metal when the mold is at rest and thereafter rotating the mold, a departure is made from the usual practice. Insofar as can be determined, heretofore the molten metal has been added when the mold was revolving and at once projected out- By first providing a pool of molten metal and then commencing the rotation of the mold, it is believed that the metalas a mass is impelled toward the circumference of the mold rather than disconnected slugs being forced therein. When.

the metal is forced in such a mass, a homogenous blade results and there is no evidence of the lack of bond displayed in castings made as first described. Following this method, one-can cast articles having extremely thin section and obtain a sound and durable product with all of the advantages naturally inherent in centrifugal castings.

avoided as any expansion circumferentially canbe taken up between the individual chills which fit within the ring with a small clearance. Further, it will be noted that each chill has molten metal on each of its sides. This means that there will be no tendency toward cracking or disintegration within the chill itself such as is found in most molds in which one side is exposed to the molten metal and the other side to a much cooler medium. Finally, these chills are interchangeable, and in the event of the failure of one of them, it is only necessary to replace it with a spare chill such as may be kept on hand. It is thus evident that the mold is very cheap to maintain and that it may be kept in such a state as to form perfect castings without extensive repair or machining.

After the mold has been poured, as indicated above, the clamps securing the mold ring to the spinner may be removed and the mold ring lifted therefrom and inverted. The mass therein, which comprises the cast metal and the chills, may then be freed fromthe mold ring. Suitable pull-out holes 3i are located on the outer surface 23 of each chill and are threaded to receive a pull-out tool. Ordinarily, the chill may be loosened sufliciently to draw it from the casting by tapping it slightly; but if this fails, the pull-out tool can be threaded into the mold and more pressure applied. When the chills are withdrawn, the resulting casting'may be machined to its final external dimensions. The chills may then be replaced in the mold ring as indicated above and the procedure repeated. It is obvious that this is particularly adapted to quantity production since successive castings will not only have a high finish, due to the use of the permanent molds, but will also agree within close tolerances. The matter of finish is important, inasmuch as these impellers are designed to rotate at very high speeds, and due to their shape the vane surfaces cannot be readily machined. However, these surfaces may be smoothed by injecting a suitable abrasive in air being pumped by the impeller or by using a regular bufiing wheel and it will be found that a very high finish results.

I have thus evolved a method of casting which is or particular use with the mold described; and both method and mold are of particular advantage in forming articles with thin and complex sections such as are illustrated. The advantages of each of these and of both in combination should be apparent at once as described herein.

Some changes may be made in the arrangement, construction and combination of the various parts of my improved device without departing from the spirit of my invention and it is my intention to cover by my claims such changes as may reasonably be included within the scope thereof.

I claim as my invention:

1. The method of forming impellers, fans, and the like which comprises, the steps of arranging sector mold parts upon a horizontal rotatable support having a centrally disposed reservoir, a portion of each of said mold sector conforming to the interstices between the vanes of said impeller, the remainder of each said mold sector cooperating with adjacent mold sectors to form a peripheral wall, preheating the mold, pouring an excess of molten metal into said reservoir in the central part of the mold, and then rotating the mold at a speed-suificient to impel the metal substantially instantly and continuously to the remote parts of the mold.

2. The method of forming impellers, fans, and the like which comprises, the step of arranging interchangeable mold sectors upon a horizontal rotatable support havinga reservoir therein, the inner portion of each said mold sector conforming to the interstices between the vanes of the said impeller, the outer portion of each said mold sector cooperating with adjacent mold sectors to form a peripheral wall, inner faces of said mold sectors conforming to the hub surface of said impeller, preheating the mold, pouring an excess of molten metal into the central portion of the mold and the reservoir, and then rotating the mold at a speed sufllcient to impel the metal substantially instantly and continuously to the remote parts of the mold.

3. A method of centrifugally casting articles having a central portion and a plurality of elements of reduced section extending outwardly therefrom, which comprises, the steps of arranging a mold having a central mold cavity defining said central portion and extending mold cavities connected to said central mold cavity defining said elements and having a central reservoir in communication with said central mold cavity and positioned beneath the connection between said central and extending mold cavities, pouring an excess of molten metal in said mold to form a pool therein, said pool filling said reservoir and a portion of said central mold cavity, and thereafter rotating said mold, to force said molten metal substantially instantaneously into the remote parts of said mold, said reservoir remaining filled with metal throughout the casting operation.

CHAS. E. SORENSEN.

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